I dont care so much about efficiency resistance for this mosfet, just something affordable.
These are both TO-220 packages, MXP4004AT, STF40NF20

Is there something that will just work that is common?
Here is that datasheet STF40NF20
http://www.st.com/web/en/resource/technical/document/datasheet/CD00150592.pdf

And also for MXP4004AT, is there a common cheap equivalent?
http://www.mir-power.com/mnk/uploadfile/2012-2/2012211331291.pdf

These are in Schumacher XI75DU inverters 750 watts. I got a broken lot to fix and cannibalized one to fix another. Plus there are 2 more with totally different insides but the same appearance on the outside.

 

I think these parts are far from common.
What is your definition of affordable?

On the STF40NF20
Digikey has stock and will charge you 28.55 for Qty. 10. Seems reasonable to me, but I'm retired.
http://www.digikey.com/product-search/en?x=14&y=19&lang=en&site=us&keywords=STF40NF20

Here is an alternative for $1.42 ea. in Qty. 1000
http://www.digikey.com/product-detail/en/RJK2006DPE-00#J3/RJK2006DPE-00#J3-ND/4092254
http://documentation.renesas.com/doc/products/transistor/rej03g0512_rjk2006dpj.pdf

Can't find the MXP part so I have no idea what it costs.
This one is pricier at $6.35 ea.
http://www.digikey.com/product-search/en?pv608=1915&FV=fff40015,fff8007d&k=POWER MOSFET&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=1000&ptm=0&fid=0&pageSize=25

Why are you convinced that repair is possible and/or advisable at these power levels.

 

I have lots of salvaged IRF3205 I might try for the MXP4004AT, what do you think about doing that?
http://www.irf.com/product-info/datasheets/data/irf3205.pdf

It is a do it as cheap as possible idea for me. I was looking at Ebay and prices for the STF40NF20 were much higher than what I was used to seeing for IRF mosfets.
How about even using a bigger mosfet like IRFP250N for the AC side?

http://www.irf.com/product-info/datasheets/data/irfp250n.pdf

Maybe those will fit. they are TO-247 so bigger. But I have some. Curious what would happen if these were mismatched pairs? Or say matched pairs of two slightly different mosfets?

 

I see smoke, flame, death and destruction in your future, because I don't think you posses the knowledge to do what you are doing.

The 3205 has a higher Vdss at 55V, but a smaller continuous current rating. Not a great substitute for the MXP4004AT.

Why would you buy parts on eBay. Do you really believe that you get what you pay for there? If so I have some land in southern Florida you might be interested in.

I can't even say what I think of the IRFP250N because I'm not looking at a schematic and I can't see what the AC side refers to.

 

I also found this can get 10 for about $6
http://www.irf.com/product-info/datasheets/data/irfs38n20d.pdf

The input and output capacitance are higher if thinking of a sub for the STF40NF20.
If they are all replaced as a group, what would happen is I think it either works or wont work.
Ebay has been fine for me so far buying cheap china semiconductors.

I dont see smoke flames or any other such thing occurring. If it overheats, it will shut down. How many burnt up inverters flame out on the inside, I have seen lots of smoked inverters, burnt mosfets, exploded caps from abused inverters.

Having the lower current rating by about 40 amps means little to a 750 watt inverter. Only question is if it loads up a surge would those nmosfets fail. I dont know, but I have bout 20 good used ones I can try right here. Those IRF3205 were used in a 3000 watt inverter, which had double the number of DC input mosfets than these schumaker inverters. And they would be going into a much smaller 750 watt inverter.

The AC side are those mosfets on the output side of the board.
The DC side would be those mosfets on the input board.

 

One schematic is worth a thousand word essay. I cannot turn your words into a picture.

The empirical approach to repair has many adherents. It does work some of the time; it is hard to argue with that. I suggest you keep doing what you're doing until things stop working for you for one reason or another. That will be the time for you to learn the finer points.

 

This inverter may have another issue as none of the mosfets are bad. I pulled them all out of circuit and they test ok. With them out it yields an led display 'SC' 'short circuit' error.
When the mosfets were all in there, it yielded 'HiP'. Device demands more than 750 Watts continuous power.

So it goes from error Hip to error SC?

It does have a burnt smell but it has no input or output short when checked with an ohm meter.
Wonder what is going on with it. The diodes are ok, and the caps charge with the meter.

I suppose I can try and check the mosfet drivers which look to be 3 lead transistors.

 

One small bit of advice, for what it's worth. Analyze one failed unit in detail, and try to figure out what went wrong. You may find that practically all of the others failed in the same way. That's the way it worked years ago when I first started in repair. Failures seemed to come in syndromes - on particular model would invariably fail the same way. One smart guy in the shop (or somewhere) would figure out what went wrong, then after that, the repair instructions could be simplified to something that any technician could do. Like, "power but no picture in Hazeltine 1500 - remove Q22 and upgrade to xxx, remove D7 and upgrade to yyy."

One other thing I've noted in limited experience with inexpensive switching power supplies is that manufacturers will go for the lowest cost part that will do the job, and that means that the specs of the part are just barely adequate (or sometimes inadequate at the limits) to do the job, and replacing a failed part with something of inferior characteristics will just be asking for failure, again.

Hopefully I can chime in specifically on your repair later. I've got to to do the dishes right now.

 

Thanks, here are 2 pics.
They are using different designs but are the same model inverter.

this one here has a slight burnt smell, although I see nothing looking off.
I pulled all mosfets, and it went from error HiP to error SC (still had fuses in then)
Does not read shorted on the input or output side.

This next picture, I have 2 of these, one has error 'bAd', the other 'SC'.
Neither read shorted input or output side.

I have not done anything yet with the 2 last ones.
So far I fixed one inverter that had two bad mosfets. One arrived working, the other three still broken out of the five I got.

 

Does the code "HiP" mean high-power output? And does "SC" mean short circuit? My guess is that the microcontroller in the inverter is simply reading voltage, and if it sees low or zero output voltage, it decides to report short circuit.

It's a real plus that you have one working model. You could use that to compare voltages (or even better oscilloscope waveforms) with the bad ones. If it were me, I'd start at the gates of the MOSFETs, and try to determine whether the problem was with the driving signal, or after.

 

I think the control board with the switches (front cover) might be bad.
I plugged the suspect inverters control board into the good inverter's good power board and get the same original code HiP, which means load draws greater than 750 watts.
It also turns the fan on which is abnormal.

I looked at the board with a lens carefully and compared various readings with the multimeter and it seems to have identical responses. Could it be some rom chip issue?
Here it is.

 

I also fixed another one by cannibalizing 2 output mosfets.

That different looking version uses JCS650C TO-220 package mosfets.
Does anyone know what I could use instead? I need three of those.

JCS650C

http://html.alldatasheet.com/html-pdf/410935/JSMC/JCS650C-O-C-N-B/104/1/JCS650C-O-C-N-B.html
I have a lot of these for free salvaged FB38N20D I could put in 4 of those and they have a higherr current rating.

FB38N20D
http://www.irf.com/product-info/datasheets/data/irfs38n20d.pdf

It looks like they are a good match, input and output capacitance, voltage, what do you think? They have the same 200 v rating.
So I now have 3 of 5 inverters working, and might be able to get one more going.

output mosfet lower left, there are 4, and to the right of the 4 are two power diodes.

Yes!, they worked fine. So i got that one working by substituting those 4 IRF for the JCS mosfets. They also have a higher amp rating so might be an improvement.

So it leaves only the one with a bad control board, which I don't think will be fixed.
So 4 working 750 watt inverters repaired with zero expense for parts.